This invention relates to the field of polymer films of enhanced opacity and to a method of making said film structures.
It is known that thermoplastic polymers can be loaded with inert fillers, cast into films, and thereafter stretched to form oriented thermoplastic films. This statement is generally true, however, it must be realized that the particular components employed and the specific process parameters employed, particularly when control is difficult, can result in significantly different end products or significantly contribute to the success or failure of obtaining a desired result. For example, U.S. Pat. No. 4,118,438 teaches the use of some materials similar to those contemplated by the present invention, however, the object of the patentees is diameterically opposed to the object of the present invention. This reference is concerned with a transparent polypropylene film containing fine particles of an incompatible polymer dispersed therein. This film manifests bumps or surface projections caused by the dispersed particles and the patentees maintain that this gives the transparent film, non-blocking characteristics. In U.S. Pat. Nos. 3,697,367 and 3,795,720, there is disclosed a process for preparing an uniaxially oriented mixed polymer system. The resulting material has utility as a paper substitute and can be formed into fibers for making sheet paper.
U.S. Pat. No. 4,377,616, the contents of which are incorporated by reference herein, discloses an opaque biaxially oriented polymer film structure comprising a thermoplastic polymer matrix core layer possessing numerous voids, a substantial number of which contain at least one spherical void-initiating particle, and transparent thermoplastic skin layers adhering to the surfaces of the core layer. The unique structure of the core layer imparts a much higher degree of opacity, possibly due to the effects of light scattering, than that possible by the use of opacifying pigment alone. There is no disclosure or suggestion in U.S. Pat. No. 4,377,616, however, that the opacity of such core layer and skin layers could be so dramatically increased by adding minor amounts of light absorbing colored pigment to the film structure. In fact U.S. Pat. No. 4,377,616 does not even refer to light absorbing colored pigment and teaches away from its use with the statement that the presence of pigment contributes little to the opacity of the system.